The effect that they observe makes sense, implicitly, but I struggle to see a causal link here. The nucleolus is the site of ribosome synthesis, and ribosomes

The effect that they observe makes sense, implicitly, but I struggle to see a causal link here. The nucleolus is the site of ribosome synthesis, and ribosomes are needed to make proteins. Under caloric restriction, the growth pathways in the cell get turned down, and protein translation is held back until resources become available again. It makes sense that the cell would stop synthesizing ribosomes, which are HUGE structures that take lots of energy to make. So I would say that reduced protein translation is a hallmark of longevity, and the methionine restriction experiments support that. The nucleolus getting smaller is likely one of the many effects of this adaptation. I agree with the author however that this is a great and easily-measured microscopic phenotype. Perhaps this can lead to awesome assays of cellular aging, and perhaps even drug screening for compounds that are rejuvenating.